Kevin wasn't overweight, had no history of heart disease in his family, and worked out regularly — in fact, he had once been a competitive marathoner. And yet five years earlier, at the age of 46, he had suffered a mild heart attack.

I offered to try to help. Two weeks after his conference, we caught up by phone and I quickly focused in on his sleep habits. He typically went to bed around 1:00 am, he said, but there was no good reason for staying up that late. It was just habitual.

He woke up before 6:00 am, in order to be at work by 7:15. Once again, he acknowledged that there was no compelling reason to start that early. It was just something he'd always done. My suggestions were very simple. "What if, as an experiment, you chose a highly specific time to go to bed — say 11:30 pm — and you began winding down at least a half hour before that? And what if you woke up an hour later than you do now?"

Kevin did exactly that, and dutifully from day one. There is no single behavioral change we've seen in our work with thousands of executives that more quickly and powerfully influences mood, focus, and productivity than a full night's sleep.

In Kevin's case, the impact was also rapid and dramatic. In the mornings, he stopped rushing out the door and instead sat down for breakfast, spending a half hour chatting with his wife and two teenager daughters and flipping through the newspaper.

In the past, he'd find himself exhausted by mid-morning. Now, when he got to work around 8:30 or 8:45 am, he felt a higher and more consistent level of energy. In turn, he had more focus and more clarity all the way through to lunch, even if his meetings ran back to back. In the evenings, when he got home, Kevin felt less drained and more present with his family. The effects of sleeping more cascaded through his life.

After a month, we added a new behavior to his life at work. At 12 noon every day, Kevin gets up from his desk and takes a half hour walk outside. He doesn't bring along his Blackberry and he's realized the world doesn't end. Instead, he uses the time to clear his head or to think through key issues that arose in the morning meetings.

In the past, when he felt something didn't go right at one of his meetings, his inclination was to fire off an email to a direct report right away. "Ninety-five percent of my reactions were negative and counterproductive," he told me.

Now, the walk gives him time to take a step back. He's sending far fewer emails, and instead he's asking himself, "What do I need to do to get this to a better place?" By the time he addresses the issue with anyone else, he's calmed down.

Between sleeping longer, coming into work later, and taking a half hour walk, Kevin is actually working an hour and a half less each day than he did in the past. "I'm actually more productive than I used to be because I'm more attentive to the issues I should be focusing on," he says.

"In the past, I'd come in to work already feeling tired, and I'd put my energy into getting small tactical things done, rapid fire. That allowed me to feel productive without expending too much energy, but it was like eating chocolate. The satisfaction didn't last. Now I'm putting my energy into longer-lasting, strategic activities. They take more intellectual energy, but they're more satisfying, they add more value, and they're what I really should be doing as a leader."

The lesson: Two behaviors we've devalued and sometimes even demonized — rest and renewal — have a profound impact on people's productivity at work and their satisfaction across their lives.